JP2006047922A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus having an autofocus device capable of reliably reducing a focus error in accordance with wavelength characteristics of light generated from a substrate even when there appear color aberrations in an image forming optical system for autofocusing. <P>SOLUTION: The image forming apparatus is provided with means 13-19 of illuminating a substrate 11 via at least an objective lens 19, means 41-49 of receiving light L2 generated from the substrate via at least the objective lens and producing focus signals in accordance with a relative position between the substrate and the objective lens at every wavelength bands among a plurality of wavelength bands predetermined, a means 27 of preliminarily storing the offset information among focus signals generated at every wavelength bands and a means 27 of adjusting the relative position between the substrate and the objective lens based on the focus signals and the offset information on at least one wavelength band. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to an imaging apparatus having an autofocus device that automatically adjusts the focus of a substrate, and in particular, marks formed on a substrate (semiconductor wafer, liquid crystal substrate, etc.) in a manufacturing process of a semiconductor element, a liquid crystal display element, and the like The present invention relates to an imaging apparatus having an autofocus device suitable for focus adjustment when detecting the position of the lens with high accuracy.

  In manufacturing processes such as semiconductor elements and liquid crystal display elements, a circuit pattern is transferred to a resist layer through a well-known lithography process, and a circuit pattern is formed on a predetermined material film by performing processing such as etching through the resist pattern. Is transferred (pattern forming step). By repeating this pattern forming process many times, circuit patterns of various material films are stacked on a substrate (semiconductor wafer or liquid crystal substrate), and a circuit of a semiconductor element or a liquid crystal display element is formed. .

  Furthermore, in the manufacturing process described above, in order to accurately overlay circuit patterns of various material films (in order to improve product yield), the substrate is aligned before the lithography process in each pattern forming process. The registration inspection of the resist pattern on the substrate is performed after the lithography process and before the processing process. For alignment of the substrate, an alignment mark formed on the underlying layer in the previous pattern formation step is used. For overlay inspection of the resist pattern, the overlay mark formed on the resist layer in the current pattern forming process and the overlay mark formed on the underlying layer in the previous pattern forming process are used.

Also, a device for detecting the position of the alignment mark or overlay mark (generally simply referred to as “mark”) is incorporated in a device for aligning the substrate or a device for performing overlay inspection of a resist pattern on the substrate. ing.
In the position detection device, a mark to be detected is positioned in the field of view, and after automatic focus adjustment, an image of the mark is captured by an image sensor such as a CCD camera, and a predetermined image is obtained with respect to the image of the mark. By performing the processing, the position of the mark is detected. The focus adjustment described above corresponds to adjustment of the relative position between the objective lens of the imaging optical system for position detection and the substrate. After focus adjustment, the relative position is set to the in-focus position.

As an autofocus device for such a position detection device, for example, a pupil division method has been proposed (see, for example, Patent Document 1). In the conventional autofocus device, the entire wavelength band of the reflected light generated from the substrate when the substrate is illuminated is collectively received by one sensor, and the above relative position is determined based on the output signal from the sensor. A focus signal is generated to adjust the focus of the substrate.
JP 2002-40322 A

  However, in the above-described conventional autofocus device, chromatic aberration may occur in the AF imaging optical system. Chromatic aberration can be reduced to some extent in design value calculation, but there is a limit with respect to second-order dispersion. In addition, a chromatic aberration larger than the result of the design value calculation may occur due to an error in refractive index dispersion of the glass, an error in the radius of curvature of the lens, or an error in the lens center thickness.

  When the AF imaging optical system has chromatic aberration, the imaging position on the sensor changes according to the wavelength characteristics of the reflected light generated from the substrate even if the relative position is the same. The focus signal generated based on the output signal from the lens also changes. For this reason, the relative position after focus adjustment (that is, the in-focus position) changes according to the wavelength characteristic of the reflected light from the substrate, and a focus error occurs. Incidentally, it is known that the wavelength characteristic of the reflected light from the substrate depends on the film thickness of the resist layer and the underlayer of the substrate.

  An object of the present invention is to provide an imaging device having an autofocus device that can reliably reduce a focus error according to the wavelength characteristics of light generated from a substrate even if the imaging optical system for AF has chromatic aberration. It is in.

  The imaging apparatus according to claim 1, wherein the imaging apparatus includes a stage on which a substrate is placed, an illumination unit that illuminates the substrate, and an imaging unit that forms an image of the substrate. The light incident on the imaging means is branched and received, and a focus signal corresponding to the relative position between the substrate and the imaging optical system is generated for each of a plurality of predetermined wavelength bands. A signal generating unit that stores at least in-focus position offset information obtained by the focus signal generated for each of the plurality of wavelength bands with respect to a focus position at a predetermined reference light; And an adjustment unit configured to adjust a relative position between the substrate and the imaging unit based on the focus signal and the offset information in at least one of the plurality of wavelength bands. A focusing device.

According to a second aspect of the present invention, in the imaging apparatus according to the first aspect, the signal generating unit separates the light from the substrate into the plurality of wavelength bands at the same time and the spectral element. And a plurality of light receiving elements that respectively receive the light of each wavelength band, and generate the focus signal based on an output signal from each light receiving element.
According to a third aspect of the present invention, in the imaging apparatus according to the first aspect, the signal generating unit includes a spectroscopic element that extracts, in time series, each of the light in the plurality of wavelength bands from the light from the substrate. A light receiving element that receives light in each wavelength band separated by the spectroscopic element in time series, and generates the focus signal based on an output signal from the light receiving element.

  According to a fourth aspect of the present invention, in the imaging apparatus according to any one of the first to third aspects, the signal generating unit receives light in each wavelength band and the intensity of each light Intensity adjusting means for detecting the focus signal for each wavelength band, the offset information for each wavelength band, and the substrate based on the relative intensity of light received for each wavelength band. The relative position with respect to the image forming means is adjusted.

  According to the imaging device having the autofocus device of the present invention, it is possible to reliably reduce the focus error according to the wavelength characteristic of the light generated from the substrate even if the imaging optical system for AF has chromatic aberration. .

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.
(First embodiment)
Here, an image forming apparatus having the autofocus device according to the first embodiment will be described using the overlay measurement device 10 shown in FIG. 1 as an example. The overlay measurement apparatus 10 is an apparatus that performs overlay inspection of a resist pattern (not shown) of the substrate 11 in a manufacturing process of a semiconductor element, a liquid crystal display element, or the like. In the overlay inspection, the amount of displacement of the resist pattern with respect to a circuit pattern (hereinafter referred to as “underground pattern”) formed on the underlayer of the substrate 11 is measured.

  The substrate 11 is a semiconductor wafer, a liquid crystal substrate, or the like, and is in a state after exposure / development to the resist layer and before processing a predetermined material film. A large number of measurement points are prepared on the substrate 11 for overlay inspection. The positions of the measurement points are the four corners of each shot area of the substrate 11. At each measurement point, a resist mark indicating the reference position of the resist pattern and a base mark indicating the reference position of the base pattern are formed. In the following description, the registration mark and the base mark are collectively referred to as an “overlapping mark 30”.

  In the first embodiment, as the substrate 11, a standard substrate having a substantially uniform wavelength characteristic of reflectance over the entire use wavelength of the overlay measuring apparatus 10 (the wavelength of the light source 13 described later) or a general product substrate is used. It is done. In the case of a product substrate, depending on the film thickness unevenness of the resist layer and the underlayer (deposited film), the wavelength characteristics of the reflectivity may differ for each shot region (for each overlay mark 30). Many.

Next, the overlay measuring apparatus 10 will be described.
As shown in FIG. 1A, the overlay measurement apparatus 10 includes an inspection stage 12 that supports a substrate 11, an illumination optical system (13 to 19), an imaging optical system (19 to 23), and a CCD image sensor. 25, an image processing unit 26, focus detection units (41 to 49), and a stage control unit 27. Among these, the inspection stage 12, the illumination optical system (13 to 19), the focus detection unit (41 to 49), and the stage control unit 27 function as the autofocus device of the first embodiment.

Although not shown, the inspection stage 12 has a holder for supporting the substrate 11 in a horizontal state, an XY drive unit for driving the holder in the horizontal direction (XY direction), and driving the holder in the vertical direction (Z direction). And a Z drive unit. The XY drive unit and the Z drive unit are connected to the stage control unit 27.
The stage control unit 27 controls the XY driving unit of the inspection stage 12 and moves the holder in the XY direction to position the overlay mark 30 on the substrate 11 in the visual field region. Further, based on a focus signal, which will be described later, output from the focus detection units (41 to 49), the Z drive unit of the inspection stage 12 is controlled to move the holder up and down in the Z direction. By this focus adjustment, the substrate 11 can be focused on the imaging surface of the CCD imaging device 25 (details will be described later).

  The illumination optical system (13 to 19) includes a light source 13, an illumination aperture stop 14, a condenser lens 15, a field stop 16, an illumination relay lens 17, a beam splitter 18, and an optical axis O2 arranged in order along the optical axis O1. And the first objective lens 19 disposed in the position. The beam splitter 18 has a reflection / transmission surface inclined by about 45 ° with respect to the optical axis O1, and is also disposed on the optical axis O2. The optical axis O1 of the illumination optical system (13 to 19) is perpendicular to the optical axis O2 of the imaging optical system (19 to 23).

  The light source 13 emits light having a wide wavelength band (for example, white light). The center of the illumination aperture stop 14 is located on the optical axis O1, and limits the diameter of the light having the broadband wavelength emitted from the light source 13 to a specific diameter. The condenser lens 15 condenses the light from the illumination aperture stop 14. The field stop 16 is an optical element that limits the field of view of the overlay measurement apparatus 10, and has one slit 16a that is a rectangular opening as shown in FIG. The illumination relay lens 17 collimates the light from the slit 16 a of the field stop 16. The beam splitter 18 reflects light from the illumination relay lens 17 downward.

  In the above configuration, the broadband wavelength light emitted from the light source 13 uniformly illuminates the field stop 16 via the illumination aperture stop 14 and the condenser lens 15. Then, the light that has passed through the slit 16a of the field stop 16 is guided to the beam splitter 18 through the illumination relay lens 17, reflected by its reflection / transmission surface (illumination light L1), and then the first objective on the optical axis O2. Guided to the lens 19.

The first objective lens 19 receives and collects the illumination light L1 from the beam splitter 18. As a result, the substrate 11 on the inspection stage 12 is vertically illuminated by the broadband wavelength illumination light L1 transmitted through the first objective lens 19 (epi-illumination). The first objective lens 19 corresponds to an “objective lens” in the claims.
The incident angle of the illumination light L1 when entering the substrate 11 is determined by the positional relationship between the center of the illumination aperture stop 14 and the optical axis O1. Further, the incident angle range of the illumination light L1 at each point on the substrate 11 is determined by the aperture diameter of the illumination aperture stop 14. This is because the illumination aperture stop 14 is disposed on a plane conjugate with the pupil of the first objective lens 19.

Further, since the field stop 16 and the substrate 11 are in a conjugate positional relationship, a region of the surface of the substrate 11 corresponding to the slit 16a of the field stop 16 is illuminated by the illumination light L1. That is, the image of the slit 16 a is projected on the surface of the substrate 11 by the action of the illumination relay lens 17 and the first objective lens 19.
Then, the reflected light L2 is generated from the region of the substrate 11 irradiated with the illumination light L1 having the broadband wavelength described above. The wavelength characteristic of the reflected light L2 is substantially equal to the wavelength characteristic of the illumination light L1 when the substrate 11 is a standard substrate (that is, the wavelength characteristic of reflectance is substantially uniform). When the substrate 11 is a general product substrate, the wavelength characteristic of the reflected light L2 changes according to the wavelength characteristic of the reflectance of the substrate 11. The reflected light L2 from the substrate 11 is guided to an imaging optical system (19 to 23) described later.

The imaging optical system (19 to 23) includes a first objective lens 19, a second objective lens 20, a first imaging relay lens 21, an imaging aperture stop 22, and a second connection arranged in order along the optical axis O2. And an image relay lens 23. The optical axis O2 of the imaging optical system (19 to 23) is parallel to the Z direction.
A beam splitter 18 of the illumination optical system (13 to 19) is disposed between the first objective lens 19 and the second objective lens 20, and the second objective lens 20 and the first imaging relay lens 21 are connected to each other. Between them, a beam splitter 41 of a focus detection section (41 to 49) described later is arranged. The beam splitters 18 and 41 are half prisms that perform light amplitude separation.

The first objective lens 19 collimates the reflected light L2 from the substrate 11. The reflected light L 2 collimated by the first objective lens 19 passes through the beam splitter 18 and enters the second objective lens 20. The second objective lens 20 condenses the reflected light L2 from the beam splitter 18 on the primary imaging surface 10a.
The beam splitter 41 of the focus detection unit (41 to 49) arranged at the subsequent stage of the primary image formation surface 10a includes the optical axis O3 of the focus detection unit (41 to 49) and the light of the imaging optical system (19 to 23). The reflection / transmission surface is inclined by approximately 45 ° with respect to the axis O2. The beam splitter 41 transmits a part (L3) of the reflected light L2 from the second objective lens 20 and reflects the remaining part (L4). A part of the light L3 transmitted through the beam splitter 41 is guided to the first imaging relay lens 21 of the imaging optical system (19 to 23).

The first imaging relay lens 21 collimates the light L3 from the beam splitter 41. The imaging aperture stop 22 is disposed on a plane conjugate with the pupil of the first objective lens 19, and limits the diameter of light from the first imaging relay lens 21 to a specific diameter. The second imaging relay lens 23 re-images the light from the imaging aperture stop 22 on the imaging surface (secondary imaging surface) of the CCD image sensor 25.
The CCD image pickup device 25 is an area sensor in which a plurality of pixels are two-dimensionally arranged, picks up an image (reflected image) based on the reflected light L <b> 2 from the substrate 11, and outputs an image signal to the image processing unit 26. The image signal represents a distribution (luminance distribution) related to the luminance value for each pixel on the imaging surface of the CCD imaging device 25.

  When the product substrate is placed on the inspection stage 12 as the substrate 11 and the overlay mark 30 of the product substrate is positioned in the visual field region, the image processing unit 26 determines the luminance of the image signal obtained from the CCD image sensor 25. Based on the distribution, the product substrate overlay inspection (inspection of the resist pattern overlay state with respect to the base pattern) is performed. In this case, visual observation with a television monitor (not shown) is also possible.

Further, the image processing unit 26 receives an image signal from the CCD image pickup device 25 when a standard substrate as the substrate 11 is placed on the inspection stage 12 and the overlay mark 30 of the standard substrate is positioned in the visual field region. Output to a television monitor (not shown). In this case, visual observation of the overlay mark 30 on the standard substrate is possible.
Next, the focus detection units (41 to 49) will be described. The focus detection units (41 to 49) detect whether or not the substrate 11 on the inspection stage 12 is in focus with respect to the imaging surface of the CCD image sensor 25.

  The focus detection units (41 to 49) include a beam splitter 41, an AF first relay lens 42, a parallel plane plate 43, a pupil division mirror 44, an AF second relay lens 45, and a cylindrical lens arranged in order along the optical axis O3. 46, an image forming optical system (41 to 46), a wavelength separation element 47, AF sensors 48 (1) to (3), and signal processing units 49 (1) to (3). . The AF sensors 48 (1) to (3) are line sensors, and a plurality of pixels are arranged one-dimensionally on the imaging surface 48a. The cylindrical lens 46 has a refractive power in a direction perpendicular to the pixel arrangement direction (A direction in the figure) on each imaging surface 48 a of the AF sensors 48 (1) to (3).

  In the focus detection units (41 to 49), a part of the light L4 reflected by the beam splitter 41 (hereinafter referred to as “AF light”) is collimated by the AF first relay lens 42, passes through the parallel plane plate 43, and The light enters the pupil division mirror 44. On the pupil division mirror 44, an image of the illumination aperture stop 14 of the illumination optical system (13 to 19) is formed. The plane parallel plate 43 is an optical element for adjusting the position of the image of the illumination aperture stop 14 at the center of the pupil division mirror 44, and has a mechanism capable of tilt adjustment.

  The AF light incident on the pupil division mirror 44 is amplitude-separated into light in two directions there, and then enters the wavelength separation element 47 via the AF second relay lens 45 and the cylindrical lens 46. The AF light from the cylindrical lens 46 is wavelength-separated into light L5 to L7 in three directions via the wavelength separation element 47, and then is near each imaging surface 48a of the AF sensors 48 (1) to (3). Focused. At this time, two light source images are formed on each imaging surface 48a at positions separated along the pixel arrangement direction (A direction in the figure).

  Here, the wavelength separation element 47 is a dichroic prism (optical element) having two reflection / transmission surfaces. The two reflection / transmission surfaces are orthogonal to each other and are inclined by approximately 45 ° with respect to the optical axis O3. The first reflection / transmission surface along the traveling direction of the AF light reflects the wavelength band on the longer wave side (light L7) than the predetermined wavelength α and the wavelength band on the shorter wave side than the wavelength α (light). L6, L5). Further, the second reflection / transmission surface along the traveling direction of the AF light reflects the wavelength band (L6) on the longer wave side than the predetermined wavelength β (<α), and the wavelength on the shorter wave side than the wavelength β. The band (L5) is transmitted.

  Therefore, the AF light from the cylindrical lens 46 is divided into three wavelength bands, that is, a short wavelength band (shorter wavelength side than the wavelength β) and a central wavelength band (wavelength β) by the two reflection / transmission surfaces of the wavelength separation element 47. Are separated into a longer wavelength side and a shorter wavelength side than the wavelength α) and a longer wavelength band (longer wavelength side than the wavelength α). The light L5 in the short wavelength band enters the AF sensor 48 (1), the light L6 in the center wavelength band enters the AF sensor 48 (2), and the light L7 in the long wavelength band enters the AF sensor 48 (3). Incident.

  As described above, in the focus detectors (41 to 49), the AF light is separated into three wavelength bands by the wavelength separation element 47, and the separated light beams L5 to L7 of the respective wavelength bands are independent AF sensors 48 ( 1) to (3). The three AF sensors 48 (1) to (3) receive light L <b> 5 to L <b> 7 in each wavelength band from the wavelength separation element 47, respectively. The AF sensors 48 (1) to (3) correspond to “light receiving element” in the claims. The wavelength separation element 47 corresponds to a “spectral element”.

  Output signals from the AF sensors 48 (1) to (3) are output to the signal processors 49 (1) to (3), respectively, and individually processed. For example, in the signal processing unit 49 (1), based on the output signal from the AF sensor 48 (1), the imaging centers P1 and P2 of the two light source images formed on the imaging surface 48a of the AF sensor 48 (1). A distance (FIGS. 2A to 2C) is obtained, and a focus signal corresponding to this distance is generated. Further, in the signal processing units 49 (2) and (3), based on the output signals from the AF sensors 48 (2) and (3), the imaging centers of the two light source images formed on the respective imaging surfaces 48a. A distance between P1 and P2 is obtained, and a focus signal corresponding to the distance is generated.

  That is, in the focus detectors (41 to 49), the AF light is separated into three wavelength bands by the wavelength separation element 47 (lights L5 to L7), and a focus signal is generated for each wavelength band. This focus detection part (41-49) respond | corresponds to the "signal generation means" of a claim. The focus signals for the respective wavelength bands generated by the focus detection units (41 to 49) are output from the signal processing units 49 (1) to (3) to the subsequent stage control unit 27.

Here, the distance between the imaging centers P1 and P2 of the two light source images formed on the imaging surfaces 48a of the AF sensors 48 (1) to (3) will be described.
2A, 2 </ b> B, and 2 </ b> C, according to the relative position between the substrate 11 and the first objective lens 19 for one wavelength band among the three wavelength bands (L5 to L7). It has been shown that the distance between the imaging centers P1 and P2 of the two light source images changes. 2 (a), 2 (b), and 2 (c) show the state when the substrate 11 is in the front pin state (below the focused state), the focused state, and the rear pin state (above the focused position), respectively. Corresponds to the situation.

  As can be seen from FIGS. 2A, 2B, and 2C, the imaging centers P1 and P2 of the two light source images are closer to each other in the front pin state (a) and are closer to each other in the rear pin state (c). Leave. That is, by moving the inspection stage 12 up and down in the Z direction and changing the relative position between the substrate 11 and the first objective lens 19, the imaging centers P1 and P2 of the two light source images are arranged on the pixel surface 48a. Along and away from the direction (A direction in the figure).

  Further, with respect to a certain wavelength band (for example, a short wavelength band), the change in the distance between the imaging centers P1 and P2 of the two light source images has a one-to-one correspondence with the change in the relative position between the substrate 11 and the first objective lens 19. Correspond with. Therefore, the focus signal corresponding to the distance between the imaging centers P1 and P2 already described can be considered as a focus signal corresponding to the relative position between the substrate 11 and the first objective lens 19. As described above, the focus detection units (41 to 49) generate focus signals corresponding to the relative positions of the substrate 11 and the first objective lens 19 for each wavelength band among the three wavelength bands (L5 to L7). Then, the data is output to the subsequent stage control unit 27.

  By the way, when the AF imaging optical system (41 to 46) has chromatic aberration, the AF sensors 48 (1) to (3) have the same relative position between the substrate 11 and the first objective lens 19. Since the distances between the imaging centers P1 and P2 of the two light source images on each imaging surface 48a are different, the focus signals generated for each wavelength band based on the output signals from the AF sensors 48 (1) to (3) are used. , The offset components are included in each other.

Such an offset amount between the focus signals in each wavelength band is measured in advance as follows, and is stored as offset information in the memory (storage means) of the stage control unit 27.
The measurement of the offset amount will be described. For the measurement of the offset amount, a standard substrate having substantially uniform reflectance wavelength characteristics is used as the substrate 11. The procedure for measuring the offset amount is as follows (I) to (IV).

Procedure (I) The standard substrate is transported onto the inspection stage 12 of the overlay measuring apparatus 10, the overlay mark 30 is positioned in the visual field region, and the illumination light L1 is irradiated. At this time, the wavelength characteristic of the reflected light L2 generated from the overlay mark 30 on the standard substrate is substantially equal to the wavelength characteristic of the illumination light L1.
Procedure (II) The reflected light L2 from the overlay mark 30 on the standard substrate is taken in via the imaging optical system (19 to 23), the CCD image pickup device 25 and the image processing unit 26, and visually observed on a television monitor (not shown). Meanwhile, the inspection stage 12 is driven in the Z direction to check the focus state of the mark image. Then, the inspection stage 12 is stopped at the Z position where the focus state is the best (for example, the Z position where the contrast of the mark image is the highest). At this time, the relative position between the substrate 11 and the first objective lens 19 is set to the in-focus position.

  Procedure (III) After passing the reflected light L2 from the overlay mark 30 on the standard substrate in the order of the first objective lens 19 → second objective lens 20 → AF imaging optical system (41 to 46), the wavelength The light is separated into three wavelength bands (L5 to L7) by the separation element 47 and received by each of the three AF sensors 48 (1) to (3). Then, the signal processing units 49 (1) to (3) focus on the output signals from the AF sensors 48 (1) to (3) (the distance between the imaging centers P1 and P2 of the two light source images). A focus signal for each wavelength band at the position is generated and output to the stage control unit 27 at the subsequent stage.

Procedure (IV) The stage controller 27 compares the focus signals for each wavelength band at the in-focus position, and measures the offset amount between the focus signals. Specifically, of the three wavelength bands (L5 to L7), for example, the center wavelength band (L6) is used as a reference. Then, a difference between the focus signals of the focus signal and a short wavelength band (L5) having a center wavelength of the band (L6), the difference value including the sign of the magnitude and the offset amount O _56. Moreover, obtains a difference between the focus signals of the focus signal and a long wavelength band (L7) having a center wavelength of the band (L6), the difference value including the sign of the magnitude and the offset amount O _67.

When the measurement of the offset amounts O ₅₆ and O ₆₇ between the focus signals in the respective wavelength bands is completed as described above, the stage control unit 27 stores the offset amounts O ₅₆ and O ₆₇ in the memory. Also, the focus signal (that is, the value at the in-focus position) of the center wavelength band (L6), which is the reference for the offset amounts O ₅₆ and O ₆₇ , is stored in the memory. These pieces of information are used for autofocus control for general product substrates.

Next, auto focus control for a general product substrate will be described. This autofocus control is performed before the overlay inspection. The control procedure is as follows (i) to (iii).
Procedure (i) The product substrate is transferred onto the inspection stage 12 of the overlay measuring apparatus 10, the overlay mark 30 is positioned in the field of view, and the illumination light L1 is irradiated. In the product substrate, the wavelength characteristics of the reflectance are often different for each shot region (for each overlay mark 30) depending on the film thickness unevenness of the resist layer and the base layer (deposited film). For this reason, the wavelength characteristic of the reflected light L2 generated from the overlay mark 30 on the product substrate changes according to the wavelength characteristic of the reflectance at that location.

  Procedure (ii) After passing the reflected light L2 from the overlay mark 30 on the product substrate in the order of the first objective lens 19 → second objective lens 20 → AF imaging optical system (41 to 46), the wavelength The light is separated into three wavelength bands (L5 to L7) by the separation element 47 and received by each of the three AF sensors 48 (1) to (3). The signal processing units 49 (1) to (3) are based on output signals from the AF sensors 48 (1) to (3) (the distance between the imaging centers P1 and P2 of the two light source images). A focus signal for each wavelength band at the current Z position is generated and output to the stage control unit 27 at the subsequent stage. At this time, information regarding the strength of the output signals from the AF sensors 48 (1) to (3) is also output to the stage control unit 27.

  Incidentally, the wavelength characteristic of the reflected light L2 changes according to the wavelength characteristic of the reflectance of the overlay mark 30 on the product substrate. Further, the intensity ratio of the light beams L5 to L7 separated into the three wavelength bands by the wavelength separation element 47 changes according to the wavelength characteristics of the reflected light L2, and the amount of light received by the AF sensors 48 (1) to (3). Increases or decreases relatively, and the strength of output signals from the AF sensors 48 (1) to 48 (3) also changes. For example, if the ratio of the long wavelength band (L7) is higher than the other wavelength bands (L5, L6), the output signal from the AF sensor 48 (3) corresponding to the long wavelength band (L7) becomes strong.

Procedure (iii) The stage control unit 27 refers to the information on the strength of the output signals of the AF sensors 48 (1) to (3), and the AF with the strongest output signal among the AF sensors 48 (1) to (3). Focus on the sensor. Furthermore, among the focus signals for each wavelength band at the current Z position on the product substrate, the focus signal corresponding to the sensor of interest, the offset amounts O ₅₆ and O ₆₇ in the memory, and the center wavelength at the in-focus position in the memory Based on the focus signal in the band (L6), the inspection stage 12 is driven in the Z direction so as to cancel the offset amounts O ₅₆ and O ₆₇ , and the relative position between the substrate 11 and the first objective lens 19 is adjusted. (Focus adjustment).

For example, when the output signal of the AF sensor 48 (3) corresponding to the long wavelength band (L7) is the strongest, the focus signal corresponding to the AF sensor 48 (3), the offset amount O ₆₇ in the memory, based on the focus signal having a center wavelength band in-focus position (L6), adjust the focus so as to cancel the offset amount O _67. Similarly, when the output signal of the AF sensor 48 (1) corresponding to the short wavelength band (L5) is the strongest, the focus signal corresponding to the AF sensor 48 (1), the offset amount O ₅₆ in the memory, based of the focus signal having a center wavelength band (L6) in-focus position, adjust the focus so as to cancel the offset amount O _56. When the output signal of the AF sensor 48 (2) corresponding to the center wavelength band (L6) is the strongest, the focus signal corresponding to the AF sensor 48 (2), the offset amount (= 0), and the combination in the memory. Focus adjustment is performed based on the focus signal in the center wavelength band (L6) at the focal position.

In this way, in order to take into account the offset amounts O ₅₆ and O ₆₇ in the memory at the time of focus adjustment, the “AF sensor 48” can be used regardless of the focus signal corresponding to any of the AF sensors 48 (1) to (3). The inspection stage 12 can be stopped at a Z position where the “focus signal corresponding to (2)” matches the “focus signal in the central wavelength band (L6) at the in-focus position in the memory”. After the focus adjustment, the relative position between the substrate 11 and the first objective lens 19 is set to a predetermined in-focus position. This completes the AF control of one overlay mark 30.

Thereafter, when another overlay mark 30 on the same product substrate is positioned in the field of view and the same AF control is performed, the wavelength characteristics of the reflectance at that location are different, and the reflected light generated from the overlay mark 30 If the wavelength characteristics of L2 are different, the AF sensor (target sensor) having the strongest output signal among the AF sensors 48 (1) to (3) may change.
However, in auto-focus apparatus of the first embodiment, as already described, even if using the focus signal corresponding to any of the AF sensor 48 (1) to (3), the offset amount O _56, O ₆₇ in memory For this reason, the inspection stage 12 is stopped at the Z position where the “focus signal corresponding to the AF sensor 48 (2)” matches the “focus signal in the central wavelength band (L6) at the in-focus position in the memory”. Can be made.

  Therefore, according to the autofocus device of the first embodiment, even if the AF imaging optical system (41 to 46) has chromatic aberration, it depends on the wavelength characteristics of the reflected light L2 generated from the substrate 11 (product substrate). The focus error can be reliably reduced. As a result, it is possible to set almost the same focus state in all the overlay marks 30 on the same substrate 11 (product substrate).

  For this reason, the overlay measuring apparatus 10 can perform overlay inspection of the product substrate (inspection of the overlay state of the resist pattern with respect to the base pattern) in substantially the same focus state. Incidentally, in the overlay inspection, the positions of the registration mark and the background mark are detected from the image of the overlay mark 30 and the relative positional deviation amount (superposition deviation amount Δ) is calculated, and then the obtained overlay is obtained. This is done by correcting the misalignment amount Δ with an error component (TIS value: Tool Induced Shift) caused by the apparatus. The amount of overlay deviation after correction by the TIS value becomes the final value.

  In addition, since the overlay measurement apparatus 10 can perform overlay inspection of product substrates in substantially the same focus state, variations in TIS values in one product substrate are extremely small. Therefore, when the overlay deviation amount Δ is corrected by the TIS value, it is possible to reuse the common TIS value for all the overlay marks 30 on the same product substrate. In this case, the measurement with the substrate 11 facing in the opposite direction (180 degree direction) may be omitted, and only the measurement with the substrate 11 facing in the positive direction (0 degree direction) may be performed. Throughput of overlay inspection is improved.

In the first embodiment described above, the prism-shaped wavelength separation element 47 is incorporated on the optical path of the AF imaging optical system (41 to 46). However, the present invention is not limited to this. In addition, an optical element having a wavelength separation function such as a dichroic mirror may be configured in two stages. Further, the configuration is not limited to the configuration in which the two reflection / transmission surfaces are sequentially arranged along the traveling direction of the AF light as in the wavelength separation element 47, and a cross dichroic prism may be used.
(Second Embodiment)
In the second embodiment, an overlay measurement apparatus 50 shown in FIG. 3 will be described as an example. The imaging apparatus having the autofocus device of the second embodiment includes a wavelength separation element 47, AF sensors 48 (1) to (3), and signal processing units 49 (1) to (3) of the autofocus device of the first embodiment. ), A wavelength band switching unit 51, one AF sensor 52, and a signal processing unit 53 shown in FIG. 3 are provided. Since the other configuration is the same as that of the first embodiment, the description thereof is omitted. In the overlay measurement apparatus 50 shown in FIG. 3, the inspection stage 12, the illumination optical system (13 to 19), the focus detection units (41 to 46, 51 to 53), and the stage control unit 27 are the autofocus of the second embodiment. Functions as a device.

  The focus detection units (41 to 46, 51 to 53) include an AF imaging optical system (41 to 46), a wavelength band switching unit 51, an AF sensor 52, and a signal processing unit 53. The wavelength band switching unit 51 is disposed between the second relay lens 45 and the cylindrical lens 46 of the imaging optical system (41 to 46). Therefore, the AF light enters the AF sensor 47 after passing through the second relay lens 45 → the wavelength band switching unit 51 → the cylindrical lens 46 in this order.

  As shown in FIG. 4, the wavelength band switching unit 51 is provided with three types of filters 54 (1) to (3) having different transmission wavelength bands. The transmission wavelength bands of the filters 54 (1) to (3) respectively correspond to the short wavelength band, the center wavelength band, and the long wavelength band described in the first embodiment. By rotating the three types of filters 54 (1) to (3) around the axis 51A, any one of the filters 54 (1) to (3) can be sequentially switched and inserted into the optical path of the AF light. it can.

  Therefore, by switching the filters 54 (1) to (3), light (L 5 to L 7) in the three wavelength bands can be sequentially selected (that is, extracted in time series) and guided to the AF sensor 52. The wavelength band switching unit 51 corresponds to a “selection unit” in the claims. The AF sensor 52 receives light in one wavelength band selected by the filters 54 (1) to (3) of the wavelength band switching unit 51 in order (that is, in time series). The output signal of the AF sensor 52 is output to the signal processing unit 53 for each wavelength band. The signal processing unit 53 generates a focus signal for each wavelength band based on the output signal from the AF sensor 52 and outputs the focus signal to the subsequent stage control unit 27.

Even in the autofocus device according to the second embodiment, when there is chromatic aberration in the AF imaging optical system (41 to 46), when the focus signals generated for each wavelength band are compared with each other, an offset component is included. Become. The offset amount between the focus signals is measured by changing the procedure (III) in the middle of the procedures (I) to (IV) to the next procedure (III ′).
Procedure (III ′) After passing the reflected light L2 from the overlay mark 30 on the standard substrate in the order of the first objective lens 19 → the second objective lens 20 → the imaging optical system (41 to 46) for AF, The wavelength band switching unit 51 selects one of the three wavelength bands (L5 to L7), and the AF sensor 52 receives the light. Further, the three types of filters 54 (1) to (3) are switched, and other wavelength bands are sequentially selected, and light is received by the AF sensor 52. Then, the signal processing unit 53 generates a focus signal for each wavelength band at the in-focus position based on the output signal from the AF sensor 52 (the distance between the imaging centers P1 and P2 of the two light source images). To the stage control unit 27.

In the second embodiment, the offset amounts O ₅₆ and O ₆₇ between the focus signals in each wavelength band are measured through the procedure (I) → the procedure (II) → the procedure (III ′) → the procedure (IV), and the result Is stored in the memory of the stage controller 27. Similarly to the above, the focus signal (that is, the value at the in-focus position) of the center wavelength band (L6) that is the reference for the offset amounts O ₅₆ and O ₆₇ is also stored in the memory. These pieces of information are used for autofocus control for general product substrates.

For the AF control procedure for the product substrate, the procedure (ii) in the middle of the above procedures (i) to (iii) is changed to the next procedure (ii ′), and the procedure (iii) is changed to the next procedure (iii). It is done by changing to ').
Step (ii ′) After passing the reflected light L2 from the overlay mark 30 on the product substrate in the order of the first objective lens 19 → the second objective lens 20 → the imaging optical system (41 to 46) for AF, The wavelength band switching unit 51 selects one of the three wavelength bands (L5 to L7), and the AF sensor 52 receives the light. Further, the three types of filters 54 (1) to (3) are switched, and other wavelength bands are sequentially selected, and light is received by the AF sensor 52. Based on the output signal from the AF sensor 52 (the distance between the imaging centers P1 and P2 of the two light source images), the signal processing unit 53 outputs a focus signal for each wavelength band at the current Z position of the product substrate. It is generated and output to the subsequent stage control unit 27. At this time, information regarding the strength of the output signal of the AF sensor 52 for each wavelength band is also output to the stage controller 27.

Procedure (iii ′) The stage control unit 27 refers to the information on the strength of the output signal of the AF sensor 52 for each wavelength band, and sets the output signal having the strongest output signal among the three wavelength bands (L5 to L7). Focus on it. Furthermore, among the focus signals for each wavelength band at the current Z position on the product substrate, the focus signal corresponding to the wavelength band of interest, the offset amounts O ₅₆ and O ₆₇ in the memory, and the center at the in-focus position in the memory Based on the focus signal in the wavelength band (L6), the inspection stage 12 is driven in the Z direction so as to cancel the offset amounts O ₅₆ and O ₆₇ , and the relative position between the substrate 11 and the first objective lens 19 is adjusted. Yes (focus adjustment).

As described above, in the second embodiment as well, the focus signals corresponding to any of the three wavelength bands (L5 to L7) are used in consideration of the offset amounts O ₅₆ and O ₆₇ in the memory at the time of focus adjustment. , The inspection stage 12 at a Z position such that the “focus signal corresponding to when the center wavelength band (L6) is selected” matches the “focus signal of the center wavelength band (L6) at the in-focus position in the memory”. Can be stopped. After the focus adjustment, the relative position between the substrate 11 and the first objective lens 19 is set to a predetermined in-focus position. This completes the AF control of one overlay mark 30.

  Therefore, according to the autofocus device of the second embodiment, even if the AF imaging optical system (41 to 46) has chromatic aberration, it depends on the wavelength characteristic of the reflected light L2 generated from the substrate 11 (product substrate). The focus error can be reliably reduced. As a result, it is possible to set almost the same focus state in all the overlay marks 30 on the same substrate 11 (product substrate).

  For this reason, in the overlay measuring apparatus 50, it is possible to perform overlay inspection of the product substrate in substantially the same focus state. Therefore, the variation of the TIS value in one product substrate becomes very small, and when correcting the overlay deviation amount Δ by the TIS value, the common TIS value is reused in all the overlay marks 30 on the same product substrate. Is possible. In this case, the measurement with the substrate 11 facing in the opposite direction (180 degree direction) may be omitted, and only the measurement with the substrate 11 facing in the positive direction (0 degree direction) may be performed. Throughput of overlay inspection is improved.

In the second embodiment, the wavelength band switching unit 51 is provided on the optical path of the AF imaging optical system (41 to 46). However, the present invention is not limited to this. A similar wavelength band switching unit may be provided on the optical path of the illumination optical system (13 to 19) to select the wavelength band of the illumination light L1. In the switching of the illumination wavelength, when the transmission characteristics of the illumination wavelength filter are substantially the same as any one of the filters included in the wavelength switching unit 51 of the AF imaging optical system (41 to 46), the wavelength of the AF optical system The band is set to substantially match the wavelength band of the illumination light L1.
(Modification)
In the first embodiment and the second embodiment described above, the number of wavelength band separations is three, but the number of separations may be two or four or more. As the number of separations increases, the focus error corresponding to the wavelength characteristic of the reflected light L2 generated from the substrate 11 can be greatly reduced.

  As a modified example that improves the focus accuracy, for example, the wavelength separation element may be configured by a grating or the like, and a spectrum of incident light may be received and an offset amount calculated based on the spectrum information. As the offset information, the reference wavelength or offset information with respect to the reference wavelength is obtained in advance as a wavelength-offset data table or a wavelength-offset curve. In this case, the light incident on the autofocus optical system is divided into light that is branched and incident on the grating side and light that is incident on the focus signal generation unit, and each is guided to the light receiving surface. Then, a spectrum of light from the substrate is obtained based on information received by the light receiving surface arranged on the grating side, and an offset is obtained based on this spectrum and offset information (wavelength-offset data table or wavelength-offset curve). Calculate the quantity.

  In the above-described embodiment, the offset amount is obtained based on the center wavelength band of the wavelength bands separated by the wavelength separation element 47 or the wavelength band switching unit 51, but the reference wavelength band is not limited to this. The amount of offset may be calculated based on the in-focus position in a band other than the center wavelength band, or a total reflection mirror etc. is placed on the stage and the focus position when white light is incident is used as a reference. It may be an offset amount with respect to the focus signal in each wavelength band. Alternatively, a focus position of a predetermined narrow band of wavelength light may be used as a reference.

Further, in the above-described embodiment, when measuring the offset amount between the focus signals in each wavelength band, the standard substrate having substantially uniform reflectance wavelength characteristics is used, but the present invention is not limited to this. Any known reflection characteristic that can ensure a sufficiently large intensity of the separated light of each wavelength band (L5 to L7) can be adopted as the reflection characteristic of the standard substrate.
Furthermore, in the above-described embodiment, the example in which the focus state is confirmed while visually observing the image of the overlay mark 30 in the procedure (II) when measuring the offset amount has been described, but the present invention is not limited to this. . The image processing unit 26 may perform image processing of the mark image and perform contrast AF control.

  In the above-described embodiment, the autofocus device incorporated in the overlay measurement devices 10 and 50 has been described as an example, but the present invention is not limited to this. A device that measures the amount of positional deviation between two marks formed on the same layer of the substrate 11, a device that detects an optical positional deviation between a single mark and the reference position of the camera, and a position of a single mark The present invention can also be applied to a detection apparatus and an autofocus apparatus incorporated in an apparatus that aligns the substrate 11 before the exposure process for the substrate 11 (that is, an alignment system of the exposure apparatus). In the alignment system, the position of the alignment mark formed on the underlayer is detected, and the positional relationship between the detection result and the stage coordinate system (such as an interferometer) is obtained.

  Furthermore, the same effect can be obtained not only when the offset amount is measured by the stage control unit 27 of the overlay measuring apparatuses 10 and 50 but also when an external computer connected to the overlay measuring apparatus or the like is used. Can do.

1 is a diagram illustrating a configuration of an autofocus device according to a first embodiment incorporated in an overlay measurement device 10. FIG. It is a figure explaining the principle of operation of an autofocus device. It is a figure which shows the structure of the autofocus apparatus of 2nd Embodiment incorporated in the overlay measuring apparatus. It is a figure explaining the wavelength band switch part 51 of the auto-focus apparatus of 2nd Embodiment.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10,50 Overlay measuring apparatus 11 Substrate 12 Inspection stage 13 Light source 14 Illumination aperture stop 15 Condenser lens 16 Field stop 17 Illumination relay lens 18, 41 Beam splitter 19 First objective lens 20 Second objective lens 21 First imaging relay lens DESCRIPTION OF SYMBOLS 22 Imaging aperture stop 23 2nd imaging relay lens 25 CCD image pick-up device 26 Image processing device 27 Stage control device 30 Overlay mark 42 AF 1st relay lens 43 Parallel plane plate 44 Pupil division mirror 45 AF 2nd relay lens 46 Cylindrical Lens 47 Wavelength separation element 48 (1) to (3), 52 AF sensor 49 (1) to (3), 53 Signal processing unit 51 Wavelength band switching unit

Claims (4)

In an imaging apparatus comprising a stage on which a substrate is placed, illumination means for illuminating the substrate, and imaging means for forming an image of the substrate,
The light incident on the imaging means from the substrate is branched and received, and a focus corresponding to the relative position between the substrate and the imaging optical system is determined for each wavelength band among a plurality of predetermined wavelength bands. Signal generating means for generating a signal;
Storage means for previously storing at least in-focus position offset information obtained from the focus signal generated for each of the plurality of wavelength bands with respect to the in-focus position with a predetermined reference light;
An autofocus device comprising: an adjustment unit that adjusts a relative position between the substrate and the imaging unit based on the focus signal and the offset information in at least one of the plurality of wavelength bands An imaging device comprising: The imaging device according to claim 1.
The signal generating means includes a spectroscopic element that simultaneously separates light from the substrate into the plurality of wavelength bands, and a plurality of light receiving elements that respectively receive light of each wavelength band separated by the spectroscopic element, An image forming apparatus, wherein the focus signal is generated based on an output signal from each light receiving element. The imaging device according to claim 1.
The signal generating means receives in time series the spectral element that extracts each of the light in the plurality of wavelength bands from the light from the substrate in time series, and receives the light in each wavelength band separated by the spectral element in time series An image forming apparatus comprising: a light receiving element; and generating the focus signal based on an output signal from the light receiving element. The imaging apparatus according to any one of claims 1 to 3,
The signal generation means includes intensity detection means for receiving light for each wavelength band and detecting the intensity of each light, and the adjustment means includes a focus signal for each wavelength band and the offset for each wavelength band. An imaging apparatus, wherein the relative position between the substrate and the imaging means is adjusted based on information and the relative intensity of light received for each wavelength band.

Images